1. Field of the Invention
The present invention relates to a double-sided reclining apparatus, and more particularly to a rotary-cam type double-sided reclining apparatus in which a pair of reclining devices, mechanically linked to each other, are attached to both sides of an automotive seat, and the locking and unlocking action of each of the reclining device is made by way of a rotary motion of the rotary cam mechanically linked to a reclining lever to ensure the angular adjustment of a seat back relative to a seat cushion.
2. Description of the Prior Art
On earlier model cars, a double-sided seat reclining apparatus is ordinarily used for enhancing a holding performance of a seat back adjusted at a desired angular position and for providing a long-life reclining device. Generally, each reclining device constructing the double-sided reclining apparatus is comprised of a rotary-cam type reclining device which has a stationary base fixedly connected to seat cushion, a rotatable arm fixedly connected to a seat back and rotatably supported by the base, at least one toothed block slidably guided in the two opposing parallel side walls in the base, a reclining lever, and a rotary cam which is usually fixedly connected to the pivot shaft of the reclining lever, and which acts to mesh or unmesh the toothed block with or from the inner toothed portion of the arm by way of rotation of the rotary cam. One such conventional double-sided reclining apparatus has been disclosed in Japanese Patent Provisional Publication No. 9-28496, assigned to the assignee of the present invention and corresponding to U.S. Pat. No. 5,727,846, issued Mar. 17, 1998 to Tomonori Yoshida. The double-sided reclining apparatus disclosed in the Japanese Patent Provisional Publication No. 9-28496 is characterized by the difference between a cam profile of a rotary cam included in a locking/unlocking mechanism of a first reclining device (corresponding to an outside reclining device employing a reclining-lever handle) and a cam profile of a rotary cam included in a locking/unlocking mechanism of a second reclining device (corresponding to an inside reclining device not employing a reclining-lever handle), thereby permitting a mechanical phase-lead of the inside reclining device with respect to the outside reclining device. This mechanical phase-lead provides a stable reclining action, while insuring a correct feel of the reclining-lever action. In the double-sided reclining apparatus disclosed in the Japanese Patent Provisional Publication No. 9-28496, to ensure the slight mechanical phase-lead, the cam profile of the cam included in the inside reclining device is formed with at least one raised portion being enlarged greater than a cam profile of the cam included in the outside reclining device. The raised portion of the cam profile of the cam included in the inside reclining device is formed in such a manner as to be locally enlarged mainly radially. Such a radially enlarged, raised portion tends to produce the difference between the contact area of the cam profile of the cam included in the outside reclining device with the toothed block included in the outside relining device and the contact area of the cam profile of the cam included in the inside reclining device with the toothed block included in the inside reclining device. Particularly, the contact area of the inside-reclining-device cam profile with at least one phase-leading, radially-enlarged raised portion is less than the contact area of the usual cam profile (the outside-reclining-device cam profile with no raised portion). Thus, in case of application of heavy load, such as impact load, to the arm or to the seat back, with the outer toothed portion of the toothed block meshed with the inner toothed portion of the arm, the impression or indentation of the cam-contour surface of the toothed block, created by the raised portion, tends to increase gradually. There is a possibility that the gradually-increased impression on the cam-contour surface of the toothed block results in insufficient meshed-engagement between the outer toothed portion of the toothed block and the inner toothed portion of the arm. This reduce the mechanical locking strength of the inside reclining device not employing the reclining lever.